1. Field of the Invention
This invention relates to the art of inhibiting the staining action of certain dyes, such as FD&C food dyes, by means of high molecular weight gallotannins. More particularly, this invention relates to artificially-colored compositions, particularly foodstuffs, in which the colorant is a food-approved dye having an affinity for polyamides, wherein the artificially-colored composition also contains high molecular weight gallotannins in an amount effective to inhibit staining of polymers such as nylon, silk, wool, etc. Still another aspect of this invention relates to a method for formulating foodstuffs.
2. Description of the Prior Art
The consumer acceptance of several types of foodstuffs, such as beverages (e.g., fruit-flavored soft drinks), baked goods, candies, cake mixes, gelatins, puddings, and other highly processed foods) can be adversely affected by dye migration or staining. For example, if a processed foodstuff containing a dye approved for human consumption (in the U.S., these are generally the FD&C dyes) is packaged with a material containing a polyamide, and the dye migrates to and stains the polyamide, the resulting internal appearance of the food package can be too aesthetically displeasing to be sold, even though the packaged product is perfectly safe to eat. This problem is sometimes referred to as "color bleed".
Consumer acceptance of artificially-colored food products can also be adversely affected by stubborn stains produced by inadvertent spills on materials commonly found in homes, e.g., melamine-formaldehyde sheets on counter tops, polyamide fibers (particularly in wool or nylon carpets, clothes, including silk clothes, drapes, and other woven and non-woven materials), etc. Soft drinks are especially likely to stain clothes, counter tops, drapes and carpets, even though these drinks may contain only parts per million of the food-approved dye.
Various stain-blocking agents have been investigated in terms of their ability to block or inhibit the staining action of the ingestible, non-toxic dyestuffs used in processed foods. Some of these agents are referred to as "resist agents" and can contain sulfonated aromatic compounds. See, for example, U.S. Pat. No. 5,096,726 (Keown et al.), issued Mar. 17, 1992. Although these "resist agents" can be low in toxicity, they are typically synthetic compounds not having any close analogs in nature and, to date, have not been approved for food use. Accordingly, although some of these synthetic sulfonated aromatic compounds are used as stain-resist agents applied directly to fibrous materials such as carpets, they are not presently used in foods.
The mechanism by which "resist agents" or "stainblockers" or stain-inhibiting agents prevent stain is not fully understood, partly because the staining action of non-toxic dyestuffs has been studied in depth only rarely. One such study is reported in Chapter 4 ("Interactions of Food, Drug and Cosmetic Dyes with Nylon and Other Polyamides") by L. L. Oehrl et al., ACS Symposium No. 473, Food and Packaging Interactions II, S. J. Risch et al., Editors, American Chemical Society, 1991, pages 37 to 52. According to Oehrl et al., it is speculated that the staining action of water-soluble dyestuffs containing sulfonate groups (--SO.sub.3) or other anionic solubilizing groups is largely an acid-base reaction which results in the formation of ionic bonding. Anionic solubilizing groups such as the --SO.sub.3 radicals of FD&C dyes can, of course, exist in either the salt form (e.g., --SO.sub.3 Na) or the sulfonic acid (--SO.sub.3 H) form, but in acid media, one would expect the sulfonic acid form to predominate. The stainable substrate (material which becomes stained) can contain one or more nitrogen-containing sites capable of accepting a proton. For example, the stainable substrate can comprise a polymer having such protonarable sites in side chains, repeating units, or end groups, as in the case of the primary amine terminus of a polyamide or polypeptide, a pendent amine group attached to an amino acid unit or a melamine ring, or some other non-terminal group with a primary, secondary, or tertiary nitrogen atom with a moderately or strongly nucleophilic unbonded electron pair (including the --NH-- of a polyamide) or one or more combinations of these sites. Perhaps the most common of these sites is the primary amino group (--NH.sub.2). Because the colored (stain-causing) material which comes into contact with the stainable substrate typically has a pH less than 7 and typically contains some sulfonic acid groups, transfer of a proton from an --SO.sub.3 H group to an N-atom should be possible. Upon protonation of that N-atom, a cation is formed, and the cation can form an ionic bond with a sulfonate group of the water-soluble dyestuff. When the protonation is a direct transfer of the proton of a sulfonic acid group on the dyestuff molecule to a protonatable nitrogen of the stainable substrate, the staining action can be viewed as an acid-base reaction.
This theory of staining protonatable N-containing materials is supported by evidence showing that staining or dye uptake by the stainable substrate is maximized at a pH below about 4. However, dye uptake does not always increase as the pH decreases and may level off or even diminish slightly at a pH below about 1 or 2. Oehrl et al. account for the decrease in dye uptake at very low pH values by suggesting that, at these low pH values, each dyestuff molecule becomes more efficient in protonating nitrogen atoms, hence fewer dyestuff molecules are taken up by the substrate. The maximum number of dyestuff molecules taken up by the stainable substrate appears to be reached somewhere within the pH range of about 2 to about 4, which happens to encompass the pK.sub.a values of acids commonly used in foods, e.g., citric acid (pH.sub.a =3.13).
Oehrl et al. explain how dye uptake by the stainable substrate can be reliably measured in experiments conducted in a manner analogous to dye bath treatments; the stainable substrate is immersed for some specified period of time (e.g., one hour) in a bath containing the dyestuff; and, after removal of the substrate, the amount of dye remaining in the bath can be measured; in extreme cases &gt;60%--sometimes even &gt;80%--of the dyestuff is taken up by the stainable substrate; far less than this amount of uptake will produce a visible stain.
Mildly alkaline agents are not very suitable as stain-inhibiting agents for a variety of reasons. For example, some colored materials simply cannot be marketed unless their pH is less than 7; a typical pH range for such colored materials as fruit-flavored beverages is about 2 to about 4, which is exactly in the most dangerous pH range from the standpoint of staining with typical FD&C dyes. There is a need for stain-inhibiting agents suitable for addition to foods which have very close analogs among natural materials or are themselves extracts or components of natural materials, so that, in use, a high level of safety in edible products containing same is obtained.
Some recent work by C. Paul Malone, Robert W. Keown and others at the University of Delaware has shown that a class of polyhydroxy (including dihydroxy) aromatic ring-containing compounds has an effect in inhibiting the stain-producing action of dyes and dye-containing materials on polymeric substrates which contain protonatable nitrogen sites. Compounds of the identified polyhydroxy aromatic compounds are noted as being present in extract of natural materials. Among the myriad of compounds suggested for use as stain inhibitors are naturally occurring tannin-like substances such as tannic acid. Tannic acid is also known as an acidulant and a flavor ingredient which imparts astringency to foodstuffs.